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Ind. Eng. Chem. Res. 2008, 47, 5918–5923
Rapidly Quenched Skeletal Fe-Based Catalysts for Fischer-Tropsch Synthesis Jian G. Fan,† Bao N. Zong,*,† Xiao X. Zhang,† Xiang K. Meng,† Xu H. Mu,† Guo B. Yu,‡ Ming H. Qiao,‡ and Kang N. Fan‡ Research Institute of Petroleum Processing, Beijing 100083, People’s Republic of China and Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China
A novel, rapidly quenched skeletal Fe catalyst (RQ Fe) has been prepared by alkali leaching of the Fe50Al50 alloy solidified by the rapid quenching technique and tested in gas phase Fischer-Tropsch synthesis (FTS). Characterizations demonstrate that the RQ Fe catalyst has larger specific surface area, smaller crystallite size, and higher population of the Fe(111) surface than the conventional Raney Fe catalyst prepared from the naturally solidified Fe50Al50 alloy. As compared to Raney Fe, which has FTS activity equivalent to the precipitated Fe catalyst while higher than the fused Fe catalyst, RQ Fe is 25% more active. Promotion of the RQ Fe catalyst with Mn or K further improves the FTS activity, selectivities to alkenes and higher alkanes, as well as the catalytic stability, showing that the rapid quenching technique is promising in the preparation of skeletal Fe-based catalysts with improved FTS performance. Introduction Fe-based catalysts are of great importance in Fischer-Tropsch synthesis (FTS) of hydrocarbons from syngas due to their high FTS activity and high water-gas shift (WGS) activity which helps make up the deficit of H2 in the syngas from modern energy-efficient coal gasifiers.1,2 Although various Fe-based catalysts have been prepared and used in FTS processes, most of them can be categorized to the precipitated Fe catalyst and the fused Fe catalyst. However, the preparation procedure for the precipitated Fe catalyst is rather complicated, making quality control difficult and the catalyst expensive, whereas the fused Fe catalyst is limited by the intrinsically low surface area and, consequently, the low activity.3,4 In searching for a catalyst devoid of such shortcomings, Kim et al.5 and Lu¨ et al.6,7 reported that the Raney Fe catalyst, prepared facilely by alkali leaching of the naturally solidified FeAl alloy, can be a promising alternative for the FTS process. It has been demonstrated that the Raney Fe catalyst has a skeletal structure similar to the wellknown Raney Ni catalyst, with a significantly larger BET surface area than that of the fused Fe catalyst while comparable with the precipitated Fe catalyst. Moreover, the Raney Fe catalyst shows much shorter induction period in FTS reaction and excellent attrition resistance as compared to the precipitated and fused Fe catalysts.3,4,6 However, since the first introduction of the rapid quenching technique in the preparation of alloys with nanocrystalline or amorphous structure,8 the technique has imposed substantial impact on the fundamental understanding of materials synthesis by solidification as well as the ability to develop new catalytic materials.9 Rapidly quenched alloys are attractive as new catalysts or precursors of new catalysts, because they are rich in low-coordination sites or defects that are closely related to the catalytic activity.10 Although investigations of the textural, structural, and catalytic properties of the rapidly quenched skeletal Ni catalyst (RQ Ni) have been systematic presented,11–14 * To whom correspondence should be addressed. Tel: (+86-10) 82368011. Fax: (+86-10)-82368011. E-mail: zongbn@ripp-sinopec. com. † Research Institute of Petroleum Processing. ‡ Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University.
no work addressing the effect of rapid quenching on the catalytic performance of the skeletal Fe catalyst in the FTS reaction has been reported. In this work, we prepared a rapidly quenched skeletal Fe catalyst (RQ Fe) by introducing the rapid quenching technique to the preparation of the Fe50Al50 alloy (Fe/Al, w/w). The textural, structural, and catalytic properties in the FTS reaction of the RQ Fe catalyst were compared with the Raney Fe catalyst prepared from the naturally solidified Fe50Al50 alloy. As it has been acknowledged that Mn and K are effective promoters in improving the FTS performance of the precipitated and fused Fe catalysts,15 their promoting effects on the RQ Fe catalyst in the FTS reaction were also examined and presented. Experimental Procedures Catalyst Preparation. The rapidly quenched Fe50Al50 alloy (RQ FeAl) was prepared by a single roller melt-spinning method at a cooling rate of ∼3 × 107 K s-1. Equal weights of metallic Fe and Al were melted and kept at 1573 K in a vacuum induction furnace for 30 min to ensure the homogeneity of the melt. Alloy ribbons with a cross section of ∼0.02 × 2 mm2 were obtained by spraying the melt onto a high-speed rotation water-cooled copper roller under the blanket of Ar. The ribbons were ground, sieved, and the 40-80-mesh fraction was used throughout the experiments. The RQ Fe catalyst was prepared as follows. A 2-gram portion of the RQ FeAl alloy was added to 12 cm3 of 8 M NaOH aqueous solution at 343 K under gentle stirring. After addition, the mixture was stirred at 343 K for 30 min for further alkali leaching. The black powders were washed thoroughly with distilled water to neutrality, followed by washing with absolute ethanol to replace the water. When removing the liquid, one must sure that a thin layer of liquid always covers the pyrophoric skeletal Fe powders. The resulting RQ Fe catalyst was stored under ethanol for activity tests and characterization purposes. The control Raney Fe catalyst was prepared by alkali leaching of a naturally solidified Fe50Al50 alloy (Jianchang Chemical Corp., China) using the same procedure. The RQ FeMn catalyst was prepared from a rapidly quenched ternary Fe45Mn5Al50 alloy (RQ FeMnAl) following the similar procedure to RQ Fe. Because K is volatile at the melting
10.1021/ie800285h CCC: $40.75 2008 American Chemical Society Published on Web 07/25/2008
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temperature of the FeAl alloy, postmodification of K on the RQ Fe was then adopted. The as-leached RQ Fe catalyst recovered from 2 g of the RQ FeAl alloy was soaked in 12 cm3 of K2CO3 aqueous solution with concentrations of 1.5, 3, and 6 wt%, respectively, and stirred gently for 60 min at 303 K. Then the supernatant was removed, and the solids were washed with absolute ethanol 3× and stored under ethanol. The resulting catalysts were labeled as K1-RQ Fe, K2-RQ Fe, and K3-RQ Fe, respectively. Catalyst Characterization. The composition was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES; Thermo Elemental IRIS Intrepid). The BET surface area and porosity were determined by N2 adsorption at 77 K using a Micromeritics Tristar3000 apparatus. Powder X-ray diffraction (XRD) patterns were acquired on a Bruker AXS D8 Advance X-ray diffractometer using Cu KR radiation (λ ) 0.15418 nm). The tube voltage was 40 kV, and the current was 40 mA. The catalyst was loaded in the in situ cell, with Ar purging the cell during the detection. Temperature programmed desorption of CO (CO-TPD) was performed on a homemade apparatus. After the catalyst was heated at 473 K for 30 min under flowing Ar (deoxygenated by an Alltech Oxytrap filter), it was cooled to room temperature before saturation chemisorption of CO by pulsed injection, as confirmed by the constant eluted peak area. The maximum desorption temperature, 850 K, was reached at a ramping rate of 20 K min-1. The CO signal (mass 28) was recorded using a Stanford SRS200 residual gas analyzer. Catalytic Test and Product Analysis. The FTS performance of the skeletal Fe-based catalyst was evaluated on a fixed-bed single-pass flow reactor with plug-flow hydrodynamics. As the alkali leached skeletal Fe-based catalysts were in the reduced state, no additional activation step was needed except for the thermal treatment at 423 K for 60 min under N2 flow to remove ethanol which protected the catalyst from air oxidation during catalyst loading. H2/CO/N2 syngas (0.47/0.47/0.06 mol; Praxair: 99.9% H2, 99.9% CO, 99.99% N2) was used as the reactant, in which N2 was used as the internal standard in product analysis. The reactant mixture was purified using activated carbon (SorbTech RL-13) to remove metal carbonyls and a molecular sieve trap (Matheson 452A) to remove water. All flows were metered using electronic mass flow controllers (Brooks 5850E). To compare the catalytic performances of different skeletal Fe-based catalysts on the same basis, reaction condition was maintained at 543 K, 1.5 MPa, and gas hourly space velocity (GHSV) of 1000 h-1. At the low GHSV used here, the high conversion of CO favors high water-gas shift (WGS) activity which helps make up the deficit of H2 in the syngas from modern energyefficient coal gasifiers,1 and the activity results are of more industrial significance. After reaction, the reactor was cooled down to room temperature under N2 atmosphere. Then the catalyst was unloaded from the reactor under the blanket of N2 to a container prefilled with ethanol for XRD characterization. Reactant and product streams were analyzed on line using an HP5890 gas chromatograph equipped with a 6-port sampling valve and two sample loops. The contents of one sample loop were injected into a cross-linked methyl silicone capillary column (HP-1, 50 m × 0.32 mm × 1.05 µm). A flame ionization detector (FID) was used to analyze the hydrocarbon products when they eluted from the capillary column. The contents of the other loop were injected into a Porapak Q packed column (15.2 × 0.318 cm). N2, CO, CO2, and light hydrocarbons eluting
Figure 1. XRD patterns of Raney Fe50Al50, RQ Fe50Al50, and RQ Fe45Mn5Al50 alloys.
from the packed column were analyzed using a thermal conductivity detector (TCD). Results and Discussion Characterization of FeAl and FeMnAl Alloys. The XRD patterns of Raney FeAl, RQ FeAl, and RQ FeMnAl alloys are shown in Figure 1. Both the naturally solidified and rapidly quenched FeAl alloys are composed of FeAl2 and Fe2Al5 phases with the diffraction peaks of the RQ FeAl alloy slightly broader than the Raney FeAl alloy.16 Moreover, Raney FeAl alloy has narrower and stronger peaks than RQ FeAl at 2θ < 30°, and in the range of 42°-45°, more separate diffraction peaks in Raney FeAl alloy are identified, whereas peaks of the RQ FeAl alloy are overlapped. These differences indicate that the RQ FeAl alloy is less crystallized than the Raney FeAl alloy. It is known that under the rapid quenching condition, the nucleation speed is much faster than the growth rate of the crystallites, thus bringing about more nuclei and finer crystallites and, consequently, the broader diffraction peaks of the RQ FeAl alloy.14 For the RQ FeMnAl alloy, there are additional diffraction peaks at 2θ of 28.1, 36.2, and 42.5° characteristic of the MnAl phase.16 In addition, a comparison of the intensities of peaks at 2θ of 19.0 and 27.9° due to the FeAl2 and Fe2Al5 phases, respectively, leads to the conclusion that the Fe2Al5/FeAl2 molar ratio in the RQ FeAl alloy is higher than that in the Raney FeAl alloy, showing that rapid quenching not only influences the crystallite size, but also modifies the phase composition of the alloy, which is anticipated to change the physicochemical and catalytic properties of the resulting skeletal Fe catalysts. Characterization of the Skeletal Fe-Based Catalysts. The bulk composition, BET surface area, pore volume, and mean pore diameter of the skeletal Fe-based catalysts are summarized in Table 1. After alkali leaching, the amount of Al decreased from 50 wt% in the original alloys to Fe(111) > Fe(110).26 Following these assignments, the peak at 766 K on RQ Fe is attributed to the recombinative desorption of CO on the Fe(111) surface of R-Fe crystallites, while the lower-lying peak at around 670 K on Raney Fe and RQ Fe is attributed to the recombinative desorption of CO on surfaces such as Fe(110) which bind more loosely with atomic C and O. Note that the (111) diffraction is systematically absent for bcc crystals such as R-Fe, so the peak does not appear in Figure 2. It is known that the Fe(111) surface has very open structure and is less stable than Fe(110) and Fe(100) surfaces.27 Although the mechanism awaits further exploration and elucidation, it is remarkable that rapid quenching can adjust the surface structure of R-Fe crystallites, which opens a new avenue for tailoring the catalytic performance of the skeletal Fe catalyst. Similar to RQ Fe, the CO-TPD profiles of RQ FeMn and K-RQ Fe catalysts all exhibit a two-peak feature as shown in Figure 3, though some slight differences are still visible. For RQ FeMn, the low-temperature peak is shifted to 682 K, while the high-temperature one is attenuated and shifted to 758 K. For K-RQ Fe, the low-temperature peak remains virtually intact, while the high-temperature one is attenuated and the maximum shifts slightly from 766 K for RQ Fe to 758 K for K3-RQ Fe. The changes in peak positions are consistent with the consensus that K and Mn can influence the interaction of CO on Fe as electronic promoters.28–31 Moreover, Figure 3 implies that Mn and K tend to occupy the more open Fe(111) surface on R-Fe crystallites, which weakens the high-temperature desorption peak. Fischer-Tropsch Synthesis Performance. The FischerTropsch synthesis performances of Raney Fe and RQ Fe catalysts are compared in Table 2 and Figure 4. It is found that the RQ Fe catalyst is always more active than the Raney Fe
Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 5921 Table 2. Catalytic Performances of Raney Fe, RQ Fe, RQ FeMn, and K-RQ Fe Catalysts in Fischer-Tropsch Synthesisa catalyst
XH2 (%)
XCO (%)
SCO2 (%)
Shydrocarbon (wt%) CH4 C2-C4 C5+
Raney Fe RQ Fe RQ FeMn K1-RQ Fe K2-RQ Fe K3-RQ Fe
44.6 56.7 60.1 62.5 63.0 52.8
68.6 86.3 93.2 96.8 97.7 83.4
34.7 35.7 35.1 36.8 37.3 40.3
15.0 17.3 13.0 12.8 12.1 13.1
38.3 40.2 36.2 28.3 31.2 29.4
46.7 42.5 50.8 58.9 56.7 57.5
C2-4)/C2-40 (weight ratio) 0.15 0.05 0.48 0.85 1.24 1.62
a Reaction condition: 543 K, 1.5 MPa, GHSV ) 1000 h-1, H2/CO ) 1, TOS ) 24 h.
Figure 5. XRD patterns of Raney Fe and RQ Fe catalysts after time on stream of 32 h in the FTS reaction.
Figure 4. CO conversion as a function of time on stream on Raney Fe, RQ Fe, RQ FeMn, and K-RQ Fe catalysts.
catalyst in FTS during the reaction period investigated. At a time on stream of 24 h, the CO conversion over Raney Fe is 68.6%, while the conversion reaches 86.3% over RQ Fe. The slightly higher selectivity to CO2, CH4, and C2-C4 alkanes, as well as the lower C2-4)/C2-40 (olefin/paraffin) weight ratio over the RQ Fe catalyst indicate that RQ Fe has slightly higher WGS activity and higher FTS activity than Raney Fe. It has been reported that the FTS activity of Raney Fe is equivalent to the conventional precipitated Fe catalyst while higher than the fused Fe catalyst,6 inferring the superior FTS activity of the RQ Fe catalyst to those conventional Fe-based catalysts. The higher FTS activity of the RQ Fe catalyst is desirable, as it allows for the improvement of the FTS selectivity using more stringent modification methods, while the activity can still be retained at an acceptable level. Figure 5 shows the XRD patterns of Raney Fe and RQ Fe catalysts after a time on stream of 32 h in the FTS reaction. In contrast to the as-leached skeletal Fe catalysts, both used catalysts are dominated by diffractions at 2θ of 35.5, 43.1, 53.5, 57.0, and 62.7°, suggesting that magnetite formed during FTS is highly crystallized, which is consistent with the observations by Lu¨7 on used Raney Fe catalyst and Bukur et al.32 and Bian et al.33 on used precipitated Fe catalysts. It is noted that although magnetite is the most active phase for the WGS reaction on Fe catalysts,34 this phase is not active for FTS, as Huang et al. evidenced that magnetite alone showed no initial activity when being exposed to syngas.35 From the broadening of the (311) peak of magnetite, the magnetite crystallite sizes of 28.3 and 23.5 nm are estimated for the used Raney Fe and RQ Fe catalysts, respectively. The smaller magnetite crystallites in the used RQ Fe may be inherited from the smaller R-Fe crystallite size in the as-leached RQ Fe. Alternatively, the occurrence of the weak but reproducible feature at 2θ of ∼44°, attributable to iron carbide (χ-Fe5C2)
which is the active phase for FTS,36 suggests that iron carbide is ill-crystallized or amorphous in the used Raney Fe and RQ Fe catalysts.32,33 The relatively stronger diffraction peak of iron carbide for the used RQ Fe catalyst, compared with that for the used Raney Fe catalyst, conforms with the higher FTS activity of the RQ Fe catalyst. Because Mn and K have been found to be the most efficient promoters for Fe-based catalysts among all of the additives studied to date, their promoting effects on the FTS performance of the RQ Fe catalyst are also investigated in the present work aiming to improve the FTS selectivity. Table 2 shows that CO conversion is improved further to 93.2% on the RQ FeMn catalyst, which can be partly related to the larger specific surface area and the smaller crystallite size of R-Fe. Table 2 also shows that the product distribution over the RQ FeMn catalyst shifts to longer alkanes and the C2-4)/C2-40 ratio increases from 0.05 to 0.48. The electronic effect of Mn, which enhances the chemisorption of CO on Fe, is responsible for the better selectivity of Mn-promoted Fe catalysts.37,38 Alternatively, Kim et al. compared the FTS performances of Raney Fe and Raney FeMn leached by alkali from a FeMnAl alloy, and observed the promoting effect of Mn on the selectivity while having an adverse effect on the activity.5 It is attractive that incorporation of Mn to skeletal Fe by the rapid quenching technique can synchronously improve the activity and selectivity of the RQ Fe catalyst in FTS reaction. Moreover, the RQ FeMn catalyst is very stable. Figure 4 shows that the CO conversion on the RQ FeMn catalyst remains constant within a time on stream (TOS) of 72 h, whereas the CO conversion decreases monotonically by ∼ 10% on Raney Fe and RQ Fe catalysts within the same time span. These results disclose the unique merits of the rapid quenching technique in promoting the Fe-based FTS catalysts. Modification of RQ FeAl alloy with other elements is being carried out in these laboratories. The promotion effect of K is qualitatively similar to that of Mn. Table 2 and Figure 4 show that with the increment of K in RQ Fe, the FTS activity increases significantly and passes through a maximum at 0.7 wt% of K for the K2-RQ Fe catalyst. Then the activity and stability decrease at higher K content. As compared to Mn, K is more effective in depleting methane and C2-C4 and in improving the selectivities to olefins and longer alkanes. The effect of K on the FTS performance of the RQ Fe catalyst observed here is in agreement with the studies on other Fe-based catalysts. Raje et al. identified that at high syngas conversion, the catalyst activity increased as the K content increased, because at high conversion, the activity is determined
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by H2 generated from the WGS reaction which is accelerated by K,39 as also evidenced by the higher CO2 selectivity shown in Table 2. When the K content is even higher, CO dissociation proceeds faster than carbon hydrogenation, leading to an excessive carbon deposition that eventually deactivates the catalyst surface,40 which explains the lower activity and the inferior stability of the K3-RQ Fe catalyst with higher K content. Although various Fe-based catalysts have been prepared and used in FTS processes, most of them can be categorized to the precipitated Fe catalyst and the fused Fe catalyst. The present work shows that the RQ Fe-based catalysts have larger specific surfaces than the fused Fe catalyst, simpler preparation procedure than the precipitated Fe catalyst, and higher FTS activity than the Raney Fe catalyst whose activity is comparable with the precipitated Fe catalyst.6 Moreover, the RQ Fe-based catalysts have much shorter induction periods (
